A screening system for screening workpieces to remove defective workpieces that do not satisfy predetermined conditions requires a conveying system that conveys workpieces from a workpiece feeding mechanism to a defective-piece inspection area.
As an example of a screening system including such a conveying system, FIG. 18 illustrates a screening system 301 described in Japanese Unexamined Patent Application Publication No. 2012-55859.
The screening system 301 according to Japanese Unexamined Patent Application Publication No. 2012-55859 conveys workpieces W, including fine workpieces GW and defective workpieces NW, from a feeding mechanism 302 to an inspection area 303.
The workpieces W are conveyed by a conveying system that includes a belt conveyor 304 and a slide 305 that guides the workpieces W fed from the feeding mechanism 302 to the belt conveyor 304.
The slide 305 has a vibrating mechanism, not illustrated, attached thereto. When the vibrating mechanism applies vibrations to the slide 305, the workpieces W move over the slide 305 and are conveyed to a front end portion of the belt conveyor 304.
Subsequently, the workpieces W are conveyed by circulation of the belt at a predetermined speed from the front end portion to a rear end portion of the belt conveyor 304.
The workpieces W that have arrived at the rear end portion of the belt conveyor 304 are thrown toward the inspection area 303 by the momentum gained while the workpieces W are conveyed by the belt conveyor 304. The inspection area 303 is a space defined by an emitting device 303a, which applies electromagnetic waves such as visible light or x-rays to the workpieces W, and a receiver 303b. 
The electromagnetic waves received by the receiver 303b vary depending on the state of the workpieces W that pass through the inspection area 303. Specifically, the workpieces W are determined whether they are fine workpieces GW or defective workpieces NW in response to information of the electromagnetic waves received by the receiver 303b. 
In the screening system 301 described in Japanese Unexamined Patent Application Publication No. 2012-55859, the workpieces W are continuously fed from the feeding mechanism 302 and conveyed without being neatly arranged. Thus, the workpieces W may be conveyed while overlapping one another over the slide 305 or the belt conveyor 304.
In such a case, if the workpieces W are thrown to the inspection area 303 while overlapping one another, the individual workpieces W fail to be uniformly irradiated with the electromagnetic waves and thus fail to be correctly determined as to whether they are fine workpieces GW or defective workpieces NW.
The problem that occurs when workpieces W are conveyed while overlapping one another as described above can occur in systems other than the screening system that sorts workpieces into fine workpieces GW and defective workpieces NW by irradiating the workpieces W with electromagnetic waves as in the case of the screening system described in Japanese Unexamined Patent Application Publication No. 2012-55859.
For example, in a screening system that inspects the appearance of workpieces using a camera or other devices during conveyance, if the workpieces are conveyed while overlapping one another, part of the workpieces fails to be visually inspected and thus the workpieces fail to be correctly determined as to whether they are fine workpieces or defective workpieces.
In a curing apparatus that causes resin to cure completely by irradiating workpieces containing a photocurable resin material that has been cured temporarily with light having a predetermined wavelength during conveyance, if the workpieces are conveyed while overlapping one another, part of the workpieces fails to be irradiated with light and thus the resin cures unevenly.
In addition, in a heat treatment apparatus that heats workpieces, to each of which another component has been temporarily fixed by thermosetting resin or solder, using an infrared-ray heater during conveyance to completely fix the component to each workpiece, if the workpieces are conveyed while overlapping one another, the workpieces fail to be evenly heated and the components fail to be fixed to the workpieces at an even strength.